Neutron detector having a radioactive vanadium emitter



P 1963 K. GSTEN INGVAR ANDERSON 3,400,289

NEUTRON DETECTOR HAVING A RADIOACTIVE VANADIUM EMITTER Filed Feb. 5,

INVENTOR karL listen Ingva Andersson EQm United States Patent 3 400 289NEUTRON DETECTOR HAVING A RADIOACTIVE VANADIUM EMITTER Karl Osten IngvarAndersson, Nykoping, Sweden, as-

signor to Aktiebolaget Atomenergi, Stockholm, Sweden, a company ofSweden Filed Feb. 3, 1966, Ser. No. 524,805 Claims priority, applicationSweden, Feb. 8, 1965, 1,587/ 65 2 Claims. (Cl. 313-61) ABSTRACT OF THEDISCLOSURE A neutron detector comprises an emitter of a metal which,when neutron irradiated, gives a radioactive substance decaying byemission of beta-radiation, e.g., rhodium or silver or vanadium, and aninsulating covering,

e.g., of alumina or magnesia or beryllia, having a thickness of about0.25 mm., the insulating covering being, in turn, surrounded by acollector of, e.g., stainless steel of about 0.25 mm. thickness, thebeta registration producing an electric potential between the emitterand the collector.

The present invention relates to neutron detectors for measuring neutronflux densities by the use of ,B-current, and is concerned with theimprovement of a detector of this type.

For measuring neutron flux within nuclear reactors it is known to usedetectors comprising a B-current emitter, i.e. an element which whenirradiated with neutrons is converted to a substance decaying byemission of [3- radiation, a collector and an insulator between theemitter and the collector, the fl-radiation producing an electricpotential between said emitter and said collector. By connecting acurrent meter instrument between the emitter and the collector a currentcan directly be measured, which is proportional to the neutron fluxdensity. However, such known detectors possess certain drawbacks in thatthey have a relatively low sensitivity and a comparatively low ratiobetween sensitivities to neutron flux and to 'y-flux. Because arelatively intense 'y-flux is present in the reactor core and thesurroundings thereof and because one wants to know the magnitude of theneutron flux densities in different points of the reactor during theoperation thereof, said ratio between the sensitivities should be ashigh as possible.

Materials with favorable properties for use as emitters are vanadium,rhodium and silver. These elements offer high neutron cross sections,short half-life and high energy beta radiation. The high cross sectionis important to obtain a detector signal which is large enough todominate over disturbing effects. The short half-life gives the detectora fast response to changes in the neutron flux. The high beta energy isan advantage if a detector with good gamma discrimination is required.

The present invention describes a design of beta emission neutrondetector where the high energy of the beta radiation is used to obtainmaximum discrimination against the gamma radiation of the character thatexists in nuclear reactors. According to the invention it has been foundthat a neutron detector with either vanadium, rhodium or silver asemitter material will have a maximum ratio between neutron and gammasensitivities if the insulation layer that surrounds the emitter isgiven a thickness such that the amount of insulator is in the range70-140 mg./cm. The unit mg./cm. indicates the Weight in milligrams of 1cm. of the insulator layer. For a specific insulator material theinsulator thickness in centimeters will be obtained by dividing theamount of insulator in mg./cm. by the density of the material inquestion in mg./cm.

The emitter is suitably rhodium, silver or vanadium and the insulatorcan be A1 0 Be O or MgO. According to a certain preferred embodiment ofthe invention the emitter is a vanadium wire having a diameter of 2 mm.and being surrounded by an insulator layer of A1 0 having a thickness of0.25 mm., which insulator in turn is surrounded by a collector ofstainless steel having a thickness of 0.25 mm.

The significance of the insulator layer thickness for the properties ofa beta emission neutron detector appears from the following conditions.Beta particles from neutron-induced radioactivity in the emitter have tobe transmitted through the insulator in order to contribute to thedetector current. The thickness of the insulator obviously has to beless than the maximum range of the beta particles and preferably evenless than the average range. Gamma radiation may contribute to thedetector current if it causes ejection of high-energy electrons from theemitter. If the gamma-induced current were to be kept small most ofthese high-energy electrons would be absorbed in the insulator and forthis result a certain minimum insulator thickness is required.

The most favourable conditions for having high neutron sensitivity andat the same time low gamma sensitivity were determined from experimentscarried out in a nuclear reactor. Insulator materials considered werealuminium oxide, beryllium oxide and magnesium oxide. For aluminiumoxide which has a density of 3.8 g./cm. the range 70-140 mg./cm.corresponds to an insulator thickness of 0.18-0.36 mm.

The invention will now be more closely described by an example of apreferred embodiment with reference to the appended drawing, which showsan axial section through a neutron detector according to the invention.

The neutron detector generally indicated 1 shown in the drawingcomprises an emitter 3 consisting of a 2.0 mm. vanadium wire which issurrounded by an insulator 5 consisting of A1 0 having a thickness of0.25 mm., the insulator 5 in turn being surrounded by a collector 7consisting of a 0.25 mm. sleeve of stainless steel. The sleeve 7 is atone end thereof closed by an end wall '9 attached by welding.

At the other end of the detector unit 1 there is connected a coaxialcable 11 which comprises an outer shell 13 of Inconel surrounding a bodyof insulating material 15 Of A1 0 containing two mutuall insulated leads17, 19, one lead 17 of which is connected to the emitter 3 by soldering.The coaxial cable 11 is connected to the collector sleeve 7 by a bushing23 surrounding the cable 11 and being inserted into the sleeve 7, saidbushing being attached by soldering or welding. At its other end thecable 11 is connected to an instrument not shown for measuring thecurrent intensity.

The function of the detector is briefly the following. The vanadium wireconsists in greater part in the isotope V which, when neutronirradiated, gives the isotope V This isotope decays with a half-life of3.76 minutes by emission of B-radiation. The fraction of the i-radiation reaching or passing through the collector gives rise to anelectric current which can be read directly and is a measure of theneutron flux density, the current density being proportional to theneutron flux density.

As mentioned above the coaxial cable 11 is provided with two interiorleads, only one of which is connected to the emitter. Gamma radiationand neutrons may induce currents in the cable leads. These currents havethe same size in the two leads. Thus by having a cable with twoidentical leads and using the difference between the currents in the twoleads as signal, the cable-induced contribution is compensated for.

The invention is of course not limited to the embodiment describedherein but a plurality of embodiments are conceivable. Thus thematerials of the different parts of the detector can be varied andinstead of the described coaxial arrangement of the parts of thedetector a plate assembly is conceivable consisting of plates of therespective parts arranged in the sequenceemitter-insulator-collector-insulator-emitter-insulator-collector, etc.The emitter and the collector can also change position, the innerelectrode being negatively charged.

What is claimed is:

1. A neutron detector for use in measuring neutron flux densities,comprising an emitter selected from a group consisting of rhodium,silver and vanadium, which emitter when neutron irradiated gives aradioactive substance decaying by emission of S-radiation, a collectorand an insulator layer between the emitter and the collector, thethickness of said insulator layer being such that the layer weighs fromabout 70 to about 140 mgs. per cm. the 3- radiation producing anelectric potential between said emitter and said collector.

2. A neutron detector according to claim 1, in which the emitteris avanadium wirehaving ai'diameter of 2 mm., which wire is surrounded by aninsulator layer of A1 0 having a thickness of 0.25 mm., which insulatorin turn is surrounded by a collector of stainless steel having athickness of 0.25,mm.

References Cited JAMES W. LAWRENCE, Primary Examiner.

R. L. JUDD, Assistant Examiner.

